This invention relates in general to hole punching apparatus and, in particular, to an apparatus for punching holes in packaging material.
More specifically, but without limitation to the particular apparatus described herein for purposes of disclosing a preferred embodiment, and the best mode presently known for carrying out the invention, this invention relates to an apparatus for punching holes in flexible film or plastic bags.
As is known to those skilled in the art, flexible plastic film or plastic bags formed from materials such as polyethylene, are frequently used to package articles such as loaves of bread. In such operations, a quantity of these bags is retained on a substantially U-shaped wicket pin with the bags being carried thereon by positioning the free ends of the wicket pin through a pair of holes formed in the upper portion of the bag. The bags may then be individually removed from the wicket pin and an article such as a loaf of bread, is placed in the bag.
In order to hold the bags on the wicket pin in a proper position so that the bags may be loaded, it is necessary to form a pair of holes in the bag through which the free ends of the wicket pin pass. In certain applications it is also advantageous to be able to quickly pull or tear the bag from the wicket pins, rather than withdrawing the bag from the wicket pins upon which the bags have been placed. To this end, provision must be made so that when the bag is pulled to be torn free from the wicket pins, the bag material will tear in a predictable manner. In this way the bag can be readily removed while maintaining the integrity of the receptacle portion thereof into which an item is placed.
Various mechanisms have been provided to punch such holes in plastic materials. Two such devices are disclosed in U.S. Pat. No. 3,550,494 "MULTIPUNCH SYSTEM FOR FLEXIBLE FILM PRODUCTS" and U.S. Pat. No. 3,973,453 "RADIAL ALIGNED PUNCH". Each of these patents disclose an apparatus for perforating flexible film products such as a plastic bag, so that the bag can be supported for releasable retention on a wicket pin by passing the arms of the wicket pin through openings formed in the plastic material. U.S. Pat. No. 3,973,453 also discloses a punch member having a single cutting line face for forming a line slit in projected continuation with the line slit forming the wicket pin receiving opening to facilitate easy removal of the bag structure from the wicket pin.
U.S. Pat. Nos. 3,654,829; 4,160,396 and 4,308,774 each disclose additional apparatus for perforating thermoplastic material utilizing a serrated knife edge to cut or punch a hole in a plastic film. Heretofore such cutters have been formed from metal and require frequent periodic sharpening so that they will be keen enough to cut through the plastic. During the cutting operation, the plastic material is supported upon a backing plate having a hole formed therein which is of a diameter larger than the cutting tool. The backing plate so formed enables the cutting tool to move through the plastic material while the material is held in position against the plate.
The use of metal cutting tools as used in the prior art is satisfactory as long as the cutting tool remains sharp, and the tool and hole in the backing plate are maintained substantially concentrically out of contact during the high speed punching operation. The hole in the backing plate must be relatively close to the same size as the punching die or cutting tool to prevent stretching of the plastic material. If proper alignment is not maintained the teeth of the cutter will be broken requiring replacement of the entire cutting head. In addition, the cutting of the thermoplastic material rapidly wears the metal cutting surfaces thus dulling the cutting tool and requiring regrinding or resharpening of the cutting edge. The wear soon becomes excessive and the tool must be replaced.